Tachyphylaxis to the hypotensive effect of clonidine and the possible mechanism of this action.

In cats under chloralose-urethan anaesthesia, tachyphylaxis developed to the hypotensive effect of clonidine. Development of this tachyphylaxis was slow in the case of intravenous low doses (10--20 micrograms/kg) and rapid after high doses (50 micrograms/kg). No tachyphylaxis developed to contractions of the nictitating membrane. The phenomenon resembles the tachyphyaxis seen after the application of indirect sympathomimetics. A tachyphylaxis developed to the latter agents inhibited the hypotensive action of clonidine which, after such an intervention, invariably turned into a vasopressor effect. Clonidine exhibited the same behaviour after pretreatment with cocaine, bretylium or reserpine. Infusion of noradrenaline (10 micrograms/kg/min) counteracted the tachyphylaxis. Thus, clonidine is assumed to exert its hypotensive effect in an indirect way, i.e. by interference with the metabolism of noradrenaline.